At present, a standardization working group of the 3rd Generation Partnership Project (3GPP) is dedicated to the research on the evolution of packet switched core network (PS-CN) and universal mobile telecommunication system terrestrial radio access network (UTRAN), for the purpose of enabling the evolved PS-CN to provide higher transmission rate and shorter transmission delay, and supporting mobility management among evolved UTRAN (E-UTRAN), global system for mobile communications enhanced data rate for GSM evolution radio access network (GERAN), UTRAN, wireless local area network (WLAN) and other non-3GPP access networks. This evolved mobile telecommunication system is called evolved packet system (EPS). FIG. 1 is an architecture diagram of the EPS system, which only illustrates some of entities related to the present invention. The entities in FIG. 1 are described as below.
Mobile station (MS)/user equipment (UE) 101: in the subsequent descriptions, the MS/UE is uniformly expressed with UE, and is also called a terminal. A terminal with the capability of accessing two kinds of wireless networks is called a dual-mode terminal. A terminal with the capability of accessing multiple kinds of wireless networks is called a multi-mode terminal. In the subsequent descriptions, the UE is a dual-mode terminal or a multi-mode terminal accessible to global system for mobile communication (GSM)/universal mobile telecommunication system (UMTS) and EPS network.
E-UTRAN 102 is an evolved radio access network, which can provide higher uplink/downlink rate, shorter transmission delay and more reliable wireless transmission. The network element contained in the E-UTRAN is evolved NodeB (eNodeB), and the E-UTRAN provides radio resources for terminal access.
Serving gateway (S-GW) and packet data network gateway (PDN GW or P-GW) 103: the S-GW is a user plane entity which is responsible for user plane data route processing. The P-GW is responsible for the gateway function of the UE accessing a packet data network (PDN). The P-GW and S-GW may be implemented in one physical entity as shown in FIG. 1, or in separate physical entities.
Mobility management entity (MME) 104 is a control plane entity and a server for temporary storage of UE/user data, which is responsible for managing and storing the context of the UE (e.g. UE/user ID, mobility management status, UE security parameter, etc.), distributing a temporary ID for a UE/user, and authenticating this UE/user when the UE currently camps on this tracking area or network managed by it.
Internet protocol (IP) multimedia subsystem (IMS) is an IP-based network architecture put forth by 3GPP. It establishes an open and flexible service environment, supports multimedia application and provides rich multimedia services for users. The IMS is an IP-based telecom network architecture and is irrelevant with access technologies. In addition to providing services for the EPS, general packet radio service (GPRS), WLAN and other packet access networks, it may also provide services for the GSM, UMTS and other mobile cellular networks.
GERAN/UTRAN 105 is radio access network for legacy GSM/UMTS network.
Mobile switching center (MSC)/visitor location register (VLR) 106 are two logically separate units, which are typically implemented in one physical node. The MSC/VLR may also be a new functional entity, MSC Server, since 3GPP R4 (Release 4) stage. In the present invention, all are uniformly expressed with the MSC/VLR.
Gateway mobile switching center (GMSC) 107 is included. Serving GPRS support node (SGSN) 108 is a control network element of the GPRS network, its main roles are to record location information of the UE(s), and forward mobile packet data between the UE and a gateway GPRS supporting node (GGSN).
When the UE camps on the GSM/UMTS network, user's basic voice services, short message services and supplementary services based on voice services will be provided through a circuit switching (CS) domain. The MSC/VLR and the GMSC in FIG. 1 are network elements belonging to the CS domain. Via the CS domain, a user may perform a voice call and other services with users in the same network or other networks (e.g. fixed telephone network and other mobile networks). IP services are provided to the users via the packet switching (PS) domain. The SGSN in FIG. 1 is a network element belonging to the PS domain. UE(s)/Users access the PDN via the PS domain. The operator's IP service network in FIG. 1 is a PDN.
In the GSM/UMTS network, the UE may perform CS and PS update and/or attachment separately to access CS and PS domains. When the UE initiates the CS and PS update and/or attachment separately, it sends request messages of Location Area(LA) Update (LAU) and Routing Area (RA) Update (RAU) to the GERAN/UTRAN (GERAN or UTRAN) respectively. After receiving the LAU request message, the GERAN/UTRAN will send the LAU request message to the MSC/VLR via an A/Iu-CS interface. After receiving the LAU request message, the MSC/VLR will initiate a process of CS domain update and/or attachment. After receiving the RAU request message, the GERAN/UTRAN will send the RAU request message to the SGSN via a Gb/Iu-PS interface. The SGSN will initiate a process of PS domain update and/or attachment after receiving the RAU request message. The method of separately performing update and/or attachment to the CS domain and the PS domain requires separate radio resources and signaling.
In order to reduce mobility management signaling overhead and radio resource occupation, in the GPRS, Gs interface is introduced between MSC/VLR and SGSN. The Gs interface is used for the establishment of the Gs association between the MSC/VLR and the SGSN (i.e., the SGSN stores the VLR number and the VLR stores the SGSN number). If the Gs association exists, the MSC/VLR will page the called UE which is in an idle state via the PS domain.
The Gs association is established in combined RA/LA Update (or attach/LAU). The combined RA/LA Update (or attach/LAU) is that when a UE which supports simultaneous attachment to the CS domain and PS domain of the GSM/UMTS network performs a RAU (or attach) in the PS domain, the UE includes an indication in the RAU (attach) message; when the SGSN performs the RAU (attach), if it determines that the network supports the Gs interface, then the SGSN sends a location update message to the MSC/VLR to trigger a CS location update procedure. As such, a CS LAU procedure was completed during a PS domain RAU procedure. Once the combined RA/LA Update (or attach/LAU) is completed, the Gs association will be established between the SGSN and the MSC/VLR.
When the MSC/VLR receives a LAU request message from the A/Iu-CS interface, Gs association information will be deleted from the MSC/VLR, and the current location area information of the UE in CS domain will be stored in the MSC/VLR.
After the UE has attached in the CS domain and PS domain of the GSM/UMTS network and the Gs association has been established, the MSC/VLR will execute paging for CS services via the SGSN. For example, when the UE is called, that is a mobile terminating call is performed, the MSC/VLR sends a paging request message for CS service to the SGSN via the Gs interface, if the state of the UE in the MSC/VLR is in idle state in the CS domain. After the SGSN receives the paging request message for CS service, it will initiate a CS paging procedure and transfer the paging message for CS service to the UE. After receiving the paging message for CS service, the UE will send a CS paging response message to the MSC/VLR. Once the CS paging response message is received, a call establishment procedure will be executed between the MSC/VLR and the UE. After the call is established, the user of the UE can start a conversation with the user who has initiated the call.
It should be noted that whether the UE supports combined RA/LA Update (or attach/LAU) or not is optional. Based on the capability of simultaneously working in the CS domain and PS domain, the UE may be classified into three categories of terminals: Class A, Class B and Class C, where only Class A terminal and Class B terminal support combined RA/LA Update (or attach/LAU). Whether the SGSN and/or the MSC/VLR support the Gs interface or not is also optional. Some of MSC/VLR and/or SGSN products don't support the Gs interface, so in some GSM/UMTS networks, the CS domain and PS domain may exist at the same time but no Gs interface is deployed in the network. In this case, the UE will perform update and/or attachment separately in the CS domain and PS domain. The CS service paging is performed via the A/Iu-CS interface.
In the GSM/UMTS network, when the MSC/VLR performs a mobile terminating call, if the MSC/VLR determines that there are ongoing CS services with the UE via the A/Iu-CS interface (that is, the UE is currently in connected mode in the CS domain), then the MSC/VLR interacts with the UE directly via the existing signaling connection of the A/Iu-CS interface without paging the UE. Otherwise, in the case there is no signaling connection between the MSC/VLR and the UE (that is, the UE is in the idle state in the CS domain), the MSC/VLR will page the UE. When there is the Gs association existing between the MSC/VLR and the SGSN, the MSC/VLR executes the CS service paging via the Gs interface; and when there is no Gs association existing between the MSC/VLR and the SGSN, the MSC/VLR executes the CS service paging via the A/Iu-CS interface.
When the UE camps on the EPS network via E-UTRAN, users' basic voice services and the supplementary services based on the voice services are provided and controlled by the IMS. The EPS system per se can not provide and control voice call services and can only provide transmission for IP data. However, during initial EPS network deployment, some operators, without deploying an IMS, may deploy EPS networks in key or hot spots where the GSM/UMTS coverage is also available. In this scenario, it is hoped that voice call services can be provided through and only through the CS domain of overlapped GSM/UMTS coverage. When the UE camps on the E-UTRAN, the UE cannot camp on the GSM/UMTS network, since the UE is single radio and it can only turn on one radio transmitter/receiver (either GSM/UMTS or Long Term Evolution(LTE)) at one time because of interference etc. So the UE cannot receive CS service paging or initiate the CS service via the GSM/UMTS network when it is camped on the LTE, thus users are unable to execute voice calls, short messages and other CS services. In order to enable CS services in such scenario, 3GPP has set up a CS Fallback (CSFB) task to work out a solution.
The current CSFB solution: SGs interface is introduced between MME and MSC/VLR. The SGs is extended based on the Gs interface. When the UE camps on the EPS network via the LTE network, the UE initiates CSFB attachment or combined TA (Tracking Area) /LA Update, and a SGs association is established between the MME and the MSC/VLR (i.e. the MME stores the MSC/VLR number or address information, and the MSC/VLR stores the MME number or address information). Subsequently, when the UE moves around within LTE coverage, the latest location information of the UE is updated to the MME. If location area or the MME changes, then the information stored in the MSC/VLR is updated, too.
When the MSC/VLR receives a LAU request message from the A/Iu-CS interface, SGs association information will be deleted, and the MSC/VLR stores the location area information of the UE in CS domain.
When CS services, e.g. a mobile terminating voice call, are processed, if the UE of a called user is camped on LTE coverage, then the MSC/VLR, after receiving the call, sends a CS paging message to the MME via the SGs interface. Once receiving the CS paging message, then the MME sends CS paging to the UE via the E-UTRAN. Once the UE received the paging for CS services, the fallback procedure will be initiated. After the fallback is finished, the UE will camp on the CS domain of the GSM/UMTS network and send a CS paging response message to the MSC/VLR. After receiving the response message, the MSC/VLR will start establishing the call and the conversation.
Before introduction of idle-mode signaling reduction (ISR), in the case there are CS services, the MSC/VLR can always find the UE by paging based on the SGs, Gs or A/Iu-CS information stored in the MSC/VLR. After the ISR is introduced, in the case that the SGs interface is supported between the MSC/VLR and the MME for the purpose that the CSFB function is supported, while the Gs interface is not supported between the SGSN and the MSC/VLR and/or a UE does not support combined RA/LA Update and that ISR is activated, when the UE camp on the EPS network, the MSC/VLR will be unable to find the UE by paging during the CS services procedures in the scenario as shown in FIG. 2 according to the current technology. ISR-related concepts will be introduced and existing problems will be described hereinafter with reference to FIG. 2.
The ISR is a mechanism to limit signaling resulting from cell-reselection between radio access technologies performed by the UE when it is in an idle mode. When the UE is in an idle mode in an area overlapped by both the GSM/UMTS network and the LTE network, it may frequently switch between GSM/UMTS and LTE access modes due to movement or changes of the intensity of radio signals or other reasons. Frequent RAU (the procedure of the UE location updating in the PS domain of the GSM/UMTS network) and TAU (the procedure of the UE location updating in the EPS network) will lead to too much signaling interactions with the network. In order to reduce the signaling load on network brought about by the above-mentioned RAU/TAU, the main idea of the ISR solution in 3GPP is that when both the UE and networks support the ISR function, context information of the UE will be stored in both the SGSN and MME, and registration information from these two networks is also stored in the UE. In this way, subsequently, when the UE switches accessing between the networks, there will be no need for TAU or RAU. In other words, subsequently, when the UE moves between the GSM/UMTS and LTE networks, if the current routing area of UE (UE location information in the PS domain of the GSM/UMTS network) has been registered and the current tracking area of the UE (UE location information in the EPS network) is in the lists of tracking area that the UE registered with the network, then the UE will not initiate the RAU and the TAU. Therefore, when there is no signaling connection between the UE and networks, the networks will not know whether the UE is camped on the LTE network or the GSM/UMTS network at the moment.
ISR activated refers to the case that UE is simultaneously registered in the EPS and GSM/UMTS PS networks. The UE keep valid registration information of the EPS and GSM/UMTS PS networks and an ISR activated flag is set in the UE context, meanwhile, both the MME and the SGSN store the UE registration information, and the ISR activated flag is set in the UE's context in them respectively. ISR non-activated refers to the case that either the MME or the SGSN will store the registration information of the UE, the UE will only have registration information of one access network, either LTE network or GSM/UMTS, in this case.
FIG. 2 is a schematic diagram illustrating the problems occurring when the MSC/VLR executes CS services paging. Under the network deployment scenario as shown in FIG. 2, the SGs interface is supported between the MSC/VLR and the MME, while the Gs interface is not supported between the SGSN and the MSC/VLR, the UE and networks support the ISR, and moreover, the ISR has been activated for the UE.
In the first step as shown in FIG. 2, the UE camps on the EPS network, and performs a CSFB attachment or combined TA/LA Update procedure. The CSFB attachment is that the UE sends an attachment request message containing CSFB indication information to the EPS, and after the MME receives the attachment request message, the MME will perform an EPS attachment procedure, meanwhile, the MME will send a location update request message to the MSC/VLR to complete the update procedure of the UE in the CS domain. In this procedure, the MME generates a tracking area list based on the current tracking area identity (TAI) and other information of the UE/user and sends an attachment acceptance message containing the tracking area list to the UE. After the UE receives the attachment acceptance message, the UE will store the tracking area list and set an identity that indicates the UE has performed an update in the EPS network. After the CSFB attachment procedure is completed, the UE will have been registered in the EPS network and the GSM/UMTS CS domain, the VLR will store the corresponding MME information, the MME will store the corresponding MSC/VLR information, and the SGs association is established between the MSC/VLR and the MME. The combined TA/LA procedure is similar to the above CSFB attachment procedure, so it will not be described in detail here.
In the second step as shown in FIG. 2, when the UE moves from the LTE access network to the GSM/UMTS network, due to the radio signal quality or other reasons, the UE will perform the cell reselection and camp on the GSM/UMTS network, as the Gs interface is not supported, the UE can not perform combined update but performs RAU and LAU separately. As shown in FIG. 2, the SGSN receives a RAU message from the Gb/Iu-PS interface, and the MSC/VLR receives a LAU message from the A/Iu-CS interface. During the RAU procedure, as the SGSN, the MME and the UE all support the ISR function, the ISR is activated, the MME remain the UE/user context and other data, the SGSN sends a RAU acceptance message including ISR activated information to the UE, and the UE sets that it has been registered in both the EPS network and the PS domain of the GSM/UMTS network. The UE now is registered in both the PS domain of the GSM/UMTS network and the EPS network, and ISR is activated. When the MSC/VLR receives the LAU message via the Mu-CS interface, the MSC/VLR will delete the SGs association, and only store the location information included in this location area update message.
Subsequently, when the UE moves from the GSM/UMTS network to the LTE network, as long as the current tracking area identity of the UE is in the list of tracking area identity that the UE registered with the network (i.e. stored in the UE's context), the UE does not initiate a TAU procedure, then in the MSC/VLR there is no SGs association information. If the user of the UE is called by other user, when the MSC/VLR needs to page the UE (i.e. the UE is in idle state in the MSC/VLR), as there is no SGs association in the MSC/VLR, the MSC/VLR will only page the UE in the GSM/UMTS network via the A/Iu-CS interface, and do not initiate CS paging in the EPS network via the SGs interface. However, since the UE can only camp on one access network (either the GSM/UMTS or the LTE) at a moment, the UE can not be found via paging in the CS domain of the GSM/UMTS network, the MSC/VLR is unable to find the UE through paging when the UE is camped on LTE access network. In other words, in this case, the user of the UE is unable to receive the CS call. So far, there is no available solution to solve the above problem.